1. Field of the Invention
The present invention relates to an image forming method using color developers, and more particularly to an image forming method using two-component full color developers.
2. Discussion of the Background
Image forming methods using electrophotography are well known. A typical image forming method using electrophotography is as follows:
(1) A photoconductor is entirely charged;
(2) The photoconductor is exposed to imagewise light to form an electrostatic latent image thereon;
(3) The electrostatic latent image is developed with a developer to form a toner image on the photoconductor;
(4) The toner image is transferred to a transfer paper; and
(5) The toner image on the transfer paper is heated and/or pressed to be fixed, resulting in formation of a hard copy.
Dry toners for use in these image forming methods typically include a binder resin and a colorant as a main component, and optionally include additives such as charge controlling agents and releasing agents.
Full color images are formed, for example, by the following method:
(1) Four photoconductors are provided and charged:
(2) Each of the photoconductors is exposed to imagewise light corresponding to a yellow, a magenta, a cyan or a black image, to form an electrostatic latent image on each of the photoconductors;
(3) The latent images are developed with a yellow, a magenta, a cyan or a black developer to form a yellow, a magenta, a cyan and a black toner image on the photoconductors;
(4) The yellow, magenta, cyan and black toner image are transferred on a transfer paper one by one to form a full color image on the transfer paper; and
(5) The full color image is fixed with heat and/or pressure, resulting in formation of a hard copy having a full color image.
The properties requisite for these toners are good fixing ability, good charging ability, good fluidity, good environmental stability, high mechanical strength, and good ability to be easily pulverized when manufactured. In addition, full color toner images are required to have high gloss and high transparency as well as the properties mentioned above.
A typical method for fixing toner images formed on receiving materials is heat roller fixing methods (a contact fixing method). In this case, when a large-size transfer paper is used as a receiving material, the following problems tend to occur:
(1) The transfer paper is wrinkled when passing through heat rollers because the pressure of the heat rollers is uneven;
(2) The balance in glosses of color toner images is not good; and
(2) Resolution of toner images deteriorates because the toner images are excessively pressed when pressed under a pressure condition of conventional image fixing methods.
On the other hand, non-contact fixing methods include flash fixing methods, in which hot air is applied to a transfer paper having toner images to fix the toner images, and oven fixing methods. Oven fixing methods are preferable for fixing full color toners.
In oven fixing methods, a problem of unbalance in glosses of fixed color toner images occurs, i.e., a black toner image has higher gloss than other color toner images, because black toner absorbs heat in an amount greater than other color toners such as yellow, magenta and cyan toners.
In attempting to solve these problems, various techniques have been proposed.
Japanese Laid-Open Patent Publication No. 9-190013 discloses that in contact or non-contact fixing methods clear images can be formed by using a toner including two binder resins whose transition temperatures are different by 5.degree. C. or less. In addition, Japanese Laid-Open Patent Publication No.6-282102 discloses that images which have good melting properties and which are useful for non-contact fixing methods can be formed by using a toner including a resin A and a resin B, wherein each of the resins A and B have a transition temperature higher than 45.degree. C., and the transition temperature of the resin A is not less than 2.5.degree. C. lower than that of the resin B.
In addition, Japanese Laid-Open Patent Publications Nos. 7-271135 and 7-319316 have disclosed that images which can easily be seen can be formed by controlling the difference of gloss between the fixed images area and the non-image area so as to be small. Further, Japanese Laid-Open Patent Publication No. 7-199583 discloses that four color toner images have uniform gloss by controlling the gloss of black toner images so as to be relatively low. Furthermore, Japanese Laid-Open Patent Publication No. 8-314300 discloses a toner including a binder resin having a specified weight average molecular weight and an image forming method which uses the toner and in which the gloss of fixed toner images is determined depending on a predetermined relationship between the gloss and fixing temperature.
However, a method by which color toner images having good image qualities, good reliability and good balance in gloss can be produced has not yet been obtained.
In particular, controlling of transparency and gloss, which are the most important properties requisite for full color toner images, is not satisfactory.
Because of these reasons, a need exists for an image forming method by which color toner images having good image qualities, good reliability and good balance in gloss can be produced.